In the liquid-phase oxidation of alkyl-substituted aromatics in an alkyl carboxylic acid as solvent, the following borderline cases can be distinguished in principle at reaction temperature in dependence on the kind of material system with regard to the aggregation state of the target product, the aromatic carboxylic acid or its acid anhydride:
1. The target product largely is completely crystalline at the reaction temperature.
2. The target product still is completely dissolved in the solvent at the reaction temperature.
While in the first case a mechanical separation of the crystalline target product can be effected by simple, for instance mechanical working methods such as filtration or centrifugation without any further intermediate steps, additional process steps are required in the second case, which lead to crystallization by a decrease in temperature of the product-containing solution, before the mechanical separation of the crystals from the solution can be effected. The same is true for material systems in which the product is present in the reactor in partly dissolved and partly crystalline form.
The material systems of virtually all liquid-phase oxidation processes in use today behave such that the solution need not be cooled to initiate or promote the crystallization of the product. However, there are also carboxylic acids whose production by means of oxidation in the liquid phase is of economic interest, but which require the above-mentioned intermediate steps for a decrease in temperature with regard to the crystallization. This involves the problem that an appreciable yield of solid product requires strong cooling. Some of the alkyl carboxylic acids used as solvent also become solid in this temperature range. Therefore, care must be taken that the temperature chosen for the crystallization of the target product, which for economic reasons should be as low as possible, definitely is higher than the crystallization temperature of the solvent.
The decrease in temperature to initiate crystallization, or for the further crystallization of the product component, can for instance be effected by indirect cooling of the solution or by a step of evaporative crystallization.
When using an indirect cooling of the solution with heat exchanger surfaces, there should be a temperature difference between the cooling surface and the solution, so that heat can be withdrawn from the solution. This temperature difference should not be chosen as small as desired, since otherwise the heat exchanging surfaces become very large, which is detrimental to the economic efficiency. Therefore, there is the problem that, due to the necessity of maintaining a distance between the cooling surface temperature and the crystallization temperature of the solvent, the crystallization temperature must be chosen higher than theoretically possible for an economic removal of heat from the solution, which reduces the amount of reaction product obtained in crystalline form.
This difficulty can be overcome by applying the principle of crystallization by vacuum evaporation. In this method, the solvent usually is evaporated by an increase in temperature, whereby the solubility limit of the solid reaction product in the solvent is exceeded and the same is crystallized. DE 31 20 732 A1, for instance, describes a process for recovering sugar from suspensions of sugar crystals in juice by continuous, multi-stage evaporative condensation, in which the suspension successively is passed through a plurality of separate treatment spaces, where the juice is evaporated by supplying heat and withdrawn. However, this process involves disadvantages when thermally sensitive reaction products must be separated from solutions. Furthermore, the required high energy consumption for evaporating large amounts of solvent is disadvantageous from an economic point of view.